Dynamic behaviour of suction caissons

1.2 Foundations for offshore wind turbines 5
Figure 1.3: Monopile foundation.
1.2.2 Monopile Foundation
The monopile foundation consists of a large diameter monopile installed in the seabed.
The monopile is a welded steel pile with the same diameter as the lower section of
the wind turbine tower. However, the diameter may vary depending on the type of
connection between the pile and tower and the stiffness requirements of the foundation.
In Figure 1.3 the monopile is shown with a transition piece. The load from the wind
turbine is transferred to the surrounding soil by lateral earth pressure on the monopile.
The length of the pile is governed by the required lateral resistance of the surrounding
soil. The design depth (depth below seabed) of the monopile is typically between 20 - 30
m depending on water depth, wind turbine size/load and soil conditions. The monopile
is installed by hydraulic hammer (driven pile), vibratory hammer (vibrated pile) or may
be drilled into the seabed. Short comments on the concept:
Simple fabrication with welded steel pile with either welded flange or grouted flange
connection between tower and pile.
The wind turbine structure with monopile becomes flexible for large water depths
( 25 m). Results in decrease of natural frequency of the structure.
No preparations of the seabed are necessary; however, scour protection may be
required when the concept is used at sandy locations.
Requires heavy duty piling/drilling equipment (problem of sufficiently heavy hammers
for diameters grater than 5-6 m).
Foundation type is not suitable for locations with many large boulders in the seabed.
December 4, 2006